1. Field of the Invention
The present invention relates generally to the field of packaging substrate. More particularly, the present invention relates to a solder pad structure provided on the chip side of a packaging substrate and fabrication method thereof, which is capable of improving solderjoint reliability of the solder ball.
2. Description of the Prior Art
The current trend toward lighter, thinner and smaller electronic products has driven the market to pay more attention to the flip chip packaging technology. The flip chip technology has several advantages over the conventional packaging approaches and has thus become the mainstream in the packaging and testing industry. As the high-level ball type packaging such as ball grid array (BGA) or flip chip is widely employed, the demand for the packaging substrate thereof is increased.
The trace or circuit on the surface of the packaging substrate has become denser and denser. It is a major challenge to improve the stability, reliability and yield of both the product and the fabrication process, reduce the cost, and concurrently increase the trace density of the packaging substrate.
As known in the art, the back end process of fabricating the packaging substrate includes solder printing on the solder pads and thermal re-flow process (to form solder balls). The aforesaid solder pads are exposed by corresponding openings, which are also referred to as solder resist openings or S/R openings, formed in the surface insulation layer, i.e., solder resist layer.
As the trace density becomes denser, the dimension of the S/R opening shrinks. The amount of solder paste squeezed into the shrunk S/R opening becomes inadequate and can not contact with the solder pad at the bottom of the S/R opening. This results in defects such as falling out or contact failure of the solder ball during or after the re-flow process. The shrinkage of the S/R opening creates a test on the ability and limit of the solder paste printing process and apparatus.
Practically, the aforesaid defects such as falling out or contact failure of the solder ball often occur when the S/R opening shrinks to less than 85 micrometers and it has been known that some advanced products has S/R openings with dimension of about 60 micrometers or even less.
In light of the above, there is a strong need in this industry to provide an improved structure or method to solve the defects such as falling out or contact failure of the solder ball at the back end of the fabrication of the packaging substrate.